1. Field of the Invention
This invention relates generally to a thermal development device for image forming mechanisms that use a thermal development photosensitive member that produces images for mechanisms such as copiers, printers, and facsimile machines. In particular, it concerns a method of reducing wrinkling in the aforesaid thermal development photosensitive member.
2. Related Technical Art
A large variety of image forming mechanisms and methods are known. Japanese Laid-Open Patent number 62-147461 is a prior-art example of an image forming mechanism that uses a thermal development photosensitive member. Based on this publication, major sections, such as an exposure section, a thermal development section, and a pressure transfer section, are formed into one integrated unit inside the mechanism. Image formation takes place using steps such as those that follow.
First, a roller-shaped transfer-type thermal development photosensitive member is cut to a designated length. After this, image exposure takes place to form a latent image. Next, thermal development takes place using a heating cylinder. After this, the photosensitive member and image-receiving paper are placed together and pressure transfer takes place using pressure rollers. The photosensitive member is discharged to a photosensitive member disposal section through a separation mechanism. The image-receiving paper passes through a fixing mechanism and is discharged to a tray through the separation mechanism.
Among the elements listed above for thermal development, a part of a heating cylinder peripheral surface is rotated by using an endless belt. During that time, thermal development occurs by placing the photosensitive member between the heating cylinder and endless belt moving it. In addition, the endless belt is always rotating the heating cylinder peripheral surface.
In addition to the exemplary prior art endless belt system, there are also configurations in which the photosensitive member is generally heated while in contact with a heating cylinder. The heating surface, in this case, is a smooth curved cylindrical surface, which is linear and has no unevenness when viewed along the paper width cross section. The photosensitive member is heated while the cylinder-shaped heating surface rotates. Or, the cylinder remains stationary and the photosensitive member is heated while its back makes sliding contact with the heating surface.
However, the technology of the prior art described above, has problems in that it cannot produce a stable output image concentration and cannot easily produce high-quality images with a uniform concentration.
First, using a thermal development method in which the photosensitive member is placed between the endless belt and heating cylinder, a phenomenon occurs in which the higher the surrounding temperature the lower the concentration, in spite of the fact that the heating cylinder is maintained at a constant temperature. Another problem is that the longer the elapsed time after power is turned on, the lower the concentration. This is due to changes in thermal development conditions caused by temperature fluctuations, which are caused by thermal conductance conditions from the surrounding temperature and the heating cylinder. Moreover, the endless belt itself has a complicated configuration. Always maintaining it at a constant temperature means using a very complex and costly mechanism.
However, a desired image concentration can be obtained using a simple configuration and at low cost if the thermal development device heats by making contact with one surface of the heating cylinder. The reason for this is that during thermal development the temperature of the photosensitive member can be accurately maintained by not allowing the photosensitive member to contact any component other than the heating cylinder. The temperature of the heating cylinder can be appropriately controlled.
However, using the method described, sometimes unintended concentration unevenness will appear in the images. That is, in general, it is often the case that the photosensitive member is made of a resin film, such as polyethylene terephthalate, or that it uses paper as a base material. With these, major thermal transformations (expansion or shrinking) take place due to the heating. As a consequence, the photosensitive member gradually wrinkles beginning with the area that makes contact with the heating surface. The wrinkled area rises and is inadequately heated. This results in lack of uniform developing, which causes concentration unevenness in the image and wrinkled paper due to pressure transfer. These greatly deteriorate the image quality.
This invention takes these conditions into consideration and resolves the problems described above. Its objective is to offer a highly reliable thermal development device for image forming mechanisms that allows high-quality images to be obtained simply and at low cost by means of stable and uniform heating of the photosensitive member.